Conventionally, there has been a known superconducting cable using a superconducting wire that can be in a superconducting state at cryogenic temperatures as a conductor. A superconducting cable is expected as a power cable capable of transmitting a large current with low loss, and has been developed for the practical use.
An example of a superconducting cable is illustrated in FIG. 2. A superconducting cable 10 illustrated in FIG. 2 is a single-core superconducting cable, in which a cable core 11 as the single core is housed in a thermal insulation tube 12.
The cable core 11 is composed of a former 111, superconductive conductor layers 112, an electric insulating layer 113, superconducting shield layers 114, a normal conducting shield layer 115, a protecting layer 116, and the like. The superconductive conductor layers 112 are formed by winding a plurality of superconducting wire materials spirally on the former 111. Similarly, the superconducting shield layers 114 are formed by winding a plurality of superconducting wire materials spirally on the electric insulating layer 113.
Each of superconducting wire materials for forming the superconductive conductor layers 112 and the superconducting shield layers 114 has a laminated structure obtained by forming an intermediate layer, a superconducting layer, a protecting layer in this order on a tape-shaped metal substrate, for example. As a superconductor for forming the superconducting layer may be a RE-based superconductor (RE: rare earth element) showing superconductivity at a liquid nitrogen temperature (−196° C. in the atmospheric pressure) or more, for example. An yttrium-based superconductor (Y-based superconductor, hereinafter) expressed as the chemical formula YBa2Cu3O7-y is especially typical.
The thermal insulation tube 12 has a double tube structure constituted of an internal tube 121 and an external tube 122. Between the internal tube 121 and the external tube 122, a multilayer thermal insulator (Super Insulation) 123 is interposed and vacuumed. In addition, the outer periphery of the external tube 122 is covered by a corrosion-resistant layer 124 of polyvinyl chloride (PVC), polyethylene, or the like.
During a steady operation of the superconducting cable 10, a cooling medium such as liquid nitrogen is circulated inside the internal tube 121, and thus transmitted electric current flows into the superconductive conductor layers 112 at a very low temperature.
At such a portion at which the superconducting cable 10 and a practical system such as a power device are connected, terminal processing is applied using a terminal connecting part. In the terminal connecting part, an end of the superconducting cable 10 is housed in a low temperature container which serves as a low temperature part, and is connected to the practical system which serves as a normal temperature part through a current lead.
Further, at such a portion at which the superconducting cables 10 are connected, terminal processing is applied using an intermediate connecting part. In the intermediate connecting part, two superconducting cables 10 are introduced in the low temperature container, and cable cores 11 are connected inside the low temperature container.
In the superconducting cable line having the terminal connecting part or intermediate connecting part above explained, the superconducting cable 10 is cooled from the normal temperature to a liquid nitrogen temperature or is heated from the liquid nitrogen temperature to the normal temperature upon assembly or maintenance. It is known that, under such a heat cycle, the cable core 11 thermally expands and contracts at about 0.3% of the length of the superconducting cable.
Particularly, in the terminal connecting part or the intermediate connecting part, when the cable core 11 has difficulty in moving in a longitudinal direction, and when the cable core 11 thermally expands and contracts, a local stress applies to the superconducting cable 10. Further, buckling occurs in superconducting wire materials forming the superconducting conductor layers 112 and the superconducting shield layers 114, and performance of the superconducting cable 10 significantly decreases.
Hence, a technique has been proposed which absorbs thermal expansion and contraction of a cable core by connecting superconducting conductor layers and current leads using connection terminals such as braided wires having flexibility (flexible connection terminals) in the terminal connecting part (see, for example, Patent Literature 1). Further, a technique has been proposed which absorbs thermal expansion and contraction of a cable core by providing an offset to a superconducting cable in a terminal connecting part or allowing the terminal connecting part to slide in the longitudinal direction of the superconducting cable.
Furthermore, a method has been proposed which provides offset parts on both sides of an intermediate connecting part and, when distortion is produced in a superconducting cable due to thermal expansion and contraction of a cable core, cancels distortion by moving the intermediate connecting part up or down (see, for example, Patent Literature 2). The offset part means a method of laying a cable in a meandering manner so as to absorb thermal expansion and contraction of the cable.